1. Field of the Invention
The present invention relates to a PWM circuit and a PWM circuit control method for generating a pulse width modulation (PWM) signal used for performing inverter-control and the like on a motor.
2. Description of the Related Art
In general, a PWM circuit comprises: a count upper limit value setting register for setting the count upper limit value for determining a carrier period of PWM; a PWM counter for performing up/down count of clock signals; a reference value setting register for setting a comparative reference value which determines the duty ratio of the PWM signal; and a comparator which compares the count value of the PWM counter and the comparative reference value of the reference value setting register, and generates an active PWM signal when the former value exceeds the latter value. Japanese Patent Unexamined Publication No. 10-112982 discloses an example of such PWM circuit.
However, the effective pulse period of the PWM signal becomes a waveform which is symmetrical with respect to the center of the carrier period of the PWM signal. The waveform of the PWM signal is limited to such form as described above. Thus, when there is no significant change in the duty ratio by every carrier period, a large amount of higher harmonic wave component of the carrier period is to be contained in a sine wave signal which is generated based on the PWM signal. If the frequency of the higher harmonic wave component is an audio frequency, noise is generated.